Assuming you can get a screenshot from a regular iPad, just upscale the image to iPad Pro 12.9" dimensions. Apple should take it as-is.

Alternatively, you could use a service like testobject.com. See https://devicelab.testobject.com/#/devices for supported devices. (Commercial; not affiliated!) That would only make sense if your app actually makes use of the iPad Pro 12.9" screen size -- if it doesn't upscaling will do just as well.